Spontaneous formation of ordered structures – self-assembly – is ubiquitous in nature and observed on different length scales, ranging from atomic and molecular systems to micro-scale objects and living matter. Self-ordering in molecular and biological systems typically involves short-range hydrophobic and van der Waals interactions. Here, we introduce an approach to micro-scale self-assembly based on the joint action of attractive Casimir and repulsive electrostatic forces arising between charged metallic nanoflakes in a solution. This system forms a self-assembled optical Fabry-Perot microcavity with a fundamental mode in the visible range (long-range separation distance ca. 100-200 nm) and a tunable equilibrium configuration. Furthermore, by placing an excitonic material in the microcavity region, we are able to realize hybrid light-matter states (polaritons), whose properties, such as the coupling strength and the eigenstate composition, can be controlled in real-time by the concentration of ligand molecules in the solution and light pressure. These Casimir microcavities can find future use as sensitive and tunable platforms for a variety of applications, including opto-mechanics, nanomachinery, and cavity-induced polaritonic chemistry.